The effect of aminophenol isomers on the reduced graphene oxide hydrogels’ microstructure and capacitive performances

Organic Electronics 74 (2019) 179–189

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The effect of aminophenol isomers on the reduced graphene oxide hydrogels’ microstructure and capacitive performances

T

Xiangli Gaoa, Gaoyi Hana,*, Yunzhen Changa, Ying Zhanga, Honggang Lib a

Institute of Molecular Science, Key Lab. of Materials for Energy Conversion and Storage of Shanxi Province, Key Lab. of Chemical Biology and Molecular Engineering of Education Ministry, Shanxi University, Taiyuan, 030006, China b College of Materials Science and Engineering, Liaocheng University, Liaocheng, 252059, China

ARTICLE INFO

ABSTRACT

Keywords: Graphene hydrogels Aminophenol isomers Solvothermal method Supercapacitors

To enhance the reduced graphene oxide hydrogels' capacitive performance, many modifiers are used to add doped element into the materials and improve the structure. The isomers of aminophenol which contain amino and hydroxyl groups on the benzene ring are employed as structure regulator and modifier to synthesize highperformance reduced graphene oxide hydrogels. The influences of preparation conditions including ratios of graphene oxide to aminophenol isomers, temperature, reaction times on the samples’ structure and properties have been investigated. The results show that the oligomer of o-aminophenol is formed on reduced graphene oxide sheets while the molecules of p, m-aminophenol are bound to the sheets; and that the hydrogel prepared in the presence of p-aminophenol has larger specific surface area, total pore volume, specific capacitance and good rate capability (74.0% of 427.0 F g−1 retains at 100 mV s−1) than other. The cells assembled by the optimized hydrogels exhibit the energy density of 14.07 Wh kg−1 at power density of 124.3 W kg−1, and 4.96 Wh kg−1 at 6.78 kW kg−1. Furthermore, 92.2% of initial capacitance can be retained after 20,000 cycles for the cells assembled by the optimal hydrogel. This kind of material may be found its use in energy storage field.

1. Introduction Recently, supercapacitors (SCs) have shown wide utilization in products of modern society including electronic devices, memory backup systems, fuel cell vehicles, hybrid electric vehicles and energy management because they can provide higher power density or higher energy density than batteries or conventional capacitors, respectively [1–3]. However, there are great challenges to be overcome before SCs can be served as main power source, that is to say, the energy density must be greatly improved without affecting their other excellent properties [4]. The energy density of SCs mainly depends on the electrode material's intrinsic performance and microstructure, decomposition voltage of electrolyte and the designation of devices [5]. It has been proved that the electrode material's composition and microstructure are the key factors which determine the energy density of the device [6]. Now graphene (two-dimensional (2D) sp2-hybridized carbon material) is considered as one of the best choices for SCs’ materials owing to its excellent mechanical, electrical and thermal properties, feasible porous structure and high chemical stability [7,8]. However, the capacitive performance of graphene-based materials can not be well *

perfected because of the compact π–π stacking structure originated from the strong van der Waals forces among the layers [9]. Up to date, tremendous efforts have been made to elevate the capacitance of graphene-based materials via doping hetero atoms such as p or d-block elements [10–12], or introducing chemical modifier [13,14] and other active materials such as carbon nanotubes [15,16], transition metal oxides (MnO2, Fe3O4, SnO2 et al.) [17–19] and conducting polymers [20,21] on the sheets. Through covalent means, by reducing the graphene oxide (GO) in the presence of chemical modifiers, graphene-based materials can be made into various shapes. For instance, using GO as precursor, the reduced graphene oxide (rGO) papers or films, fibers and foams, hollow spheres and hydrogels or aerogels have been synthesized and their capacitive properties have been evaluated in the assembled SCs [3,7,22–28]; the results indicate that the performances of the products have been improved by introducing pseudo capacitance and alleviating agglomeration. The most typical example is as follows: Chen et al. have synthesized N-doped rGO hydrogels (rGOHs) by using organic amine as nitrogen sources and modifier, and the products have specific capacitance (Cs) of 190.1 F g−1 at current density of 10 A g−1 [25]; Zhang et al. have used o-phenylenediamine as N precursor and reductant to

Corresponding author. Tel.: +86 351 7010699; fax: +86 351 7016358. E-mail address: [email protected] (G. Han).

https://doi.org/10.1016/j.orgel.2019.07.005 Received 21 December 2018; Received in revised form 24 May 2019; Accepted 3 July 2019 Available online 04 July 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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obtain the N-doped rGO with a Cs value of 301 F g−1 at current density of 0.1 A g−1 [26]; Zou et al. have synthesized functionalized rGOHs in the presence of phenylenediamine isomers, and the effect of nitrogen content and nitrogen types on the electrochemical performance of rGOHs has been studied in detail [27]; Liu et al. have found that rGOHs prepared from GO in the presence of o, p and m-benzenediols show various Cs values and different rate-capabilities [28]. According to the results previously reported, it is clear that more effective modifiers still need to be searched to further improve rGOHs performances although a large number of chemical modifiers have been used to modify the rGOHs and improve their properties. Different from the small molecules used before, aminophenol (AP) possesses amino group and hydroxyl group on the phenyl ring, the amino group tends to react with GO sheets to make the rGO sheets N doped according to the groups’ nucleophilic nature [28–30], therefore extra capacitance can be provided by the N doped region of rGO and the hydroxyl group of aminophenol [31]. Here, we use o, p and m-aminophenol and GO as precursors to produce the rGOHs modified with aminophenol (rGOHAPs), and expect that the samples with large Cs and good rate capability can be obtained and elucidated its structural characteristics.

results shown in Figs. S1–S3, the optimized conditions for different samples were various. For example, the optimized samples of o-rGOHAPs were prepared at 140 °C for 8 h; while the samples of p-rGOHAPs and m-rGOHAPs were prepared by heating the corresponding mixtures with the same Rm value (1:1) at 180 °C for 10 h and 8 h, respectively. By using a Soxhlet extractor, the obtained rGOHAPs can be easily purified by ethanol until the ethanol solution became colorless. The rGOHAPs samples prepared with different Rm were defined as o-, p-, m-rGOHAPx, and those prepared at selected temperatures were defined as o-, p-, mrGOHAPTy and those prepared at selected hydrothermal times were defined as o-, p-, m-rGOHAPtz. At the same time, rGOH was prepared under similar condition for comparison. All obtained samples were freeze-dried in order to carry out the structural characterization. 2.3. Characterizations of the samples The microstructures of the samples were observed on a scanning electron microscope (SEM, JEOL-JSM-6701) and a transmission electron microscopy (TEM, JEOL 2010). Fourier transform infrared (FTIR) spectra were recorded in the range of 400–4000 cm−1 on BRUKER TENSOR 27 Infrared Spectrometric Analyzer. The specimens were prepared by grinding the mixture of dry powders of samples and KBr together and then the mixture was compressed into thin slices. By employing a Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm), the samples' X-ray diffraction (XRD) patterns were recorded in the 2 theta range of 5–80° at the scan speed of 5° min−1. By using 514 nm laser as incident light, JobinYvon Lab RAMHR800 microscopic confocal Raman spectrometer was used to record the Raman spectra of samples. An ESCAL-ab 220i-XL X-ray photoelectron spectrometer (XPS, VG Scientific, England) was employed to determine the samples’ surface function groups and elemental compositions (Al Ka source at 1486.6 eV). Nitrogen adsorption-desorption isotherms were measured on a physical adsorption instrument (Quantachrome ASIQM0002-5, America) at 77 K. According to the nitrogen adsorption data, the pore size distributions (PSD) and specific surface areas (SSA) were obtained by using Barrette-Joynere-Halenda (BJH) and Brunauer-Emmett-Teller (BET) methods.

2. Experimental section 2.1. Reagents and materials The isomers of aminophenol were of analytical grade and purchased from Shanghai Aladdin Corporation. The other chemical reagents used in this study were analytically pure. The GO dispersion was prepared according to the modified Hummers’ method [32] by oxidizing the natural graphite powder (NGP, 325 mesh) which was purchased from Tianjin Guangfu Research Institute. Generally, the as-prepared GO dispersion was diluted to a concentration of 10 mg mL−1 and stored in a refrigerator. 2.2. Synthesis of rGOHAPs The schematic diagram for preparing rGOHAPs from GO and aminophenol isomers was depicted in Fig. 1. Taking preparing o-rGOHAPs as an example: appropriate amount of o-aminophenol was firstly dissolved into ethanol to form the solution with concentration of 10.0 mg mL−1. Subsequently, certain volumes of GO suspension and oaminophenol ethanol solution were mixed under ultrasonic condition to form a homogeneous mixture with the mass ratio of o-AP to GO (Rm) = 1:1. The final concentration of GO was about 2.0 mg mL−1 and the mixture volume was about 5.0 mL. The mixtures were then transferred into the Teflon-sealed autoclaves and heated. According to the

2.4. Electrochemical measurements Firstly, two identical hydrogels (active material: ~1.0 mg) were immersed into 1.0 mol L−1 H2SO4 aqueous solution for 3 h to complete exchange of water by electrolyte, then the two slices of the gel were separated by a piece of filter paper adsorbed with the electrolyte to turn into sandwiched structure. Two Au foils were coated on the hydrogel and used as the current collectors; the electrochemical capacitors were

Fig. 1. Schematic diagram of rGOHAPs prepared from GO and the isomers of aminophenol. 180

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Fig. 2. The SEM images of rGOH(A), o-rGOHAP (B), p-rGOHAP (C), m-rGOHAP (D).

then sealed by plastic sheets for testing. Electrochemical performances of the cells were firstly evaluated in a potential difference window of 0–1 V via cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) testing methods on a CHI660C electrochemical workstation; the electrochemical impedance spectra (EIS) of the cells was measured with amplitude of 5 mV referring to open circuit potential in the frequencies of 105-10−2 Hz, the stabilities of the cells were also tested on the same instrument via successive CV testing. The Cs values of as-prepared hydrogels at various scan rates were calculated based on the following equation according to the CV curves [24]:

Cs =

IdV / vm V

3. Results and discussion 3.1. Structural characters of the samples The GO dispersion can be easily converted to hydrogels under solvothermal condition (Fig. 1). If small molecules containing amino simultaneously exist during the process, the -NH2 groups will react with GO's groups such as -CO, -COC- and -COOH to achieve the modification of rGO sheets. Different from the previous literatures [25,29,36], the molecules of aminophenol possess the groups of –NH2 and –OH, when the molecules are attached to the rGO sheets via the reaction between –NH2 group and GO sheets, the remained –OH group can carry out the Faradic reaction besides the redox reaction of the N-containing groups; on the other hand, the reaction between the oxygen-containing groups of GO and the aminophenol will become easy when the reaction carries out in water and ethanol mixture because water may drive the reverse reaction of ammonification. Furthermore, the conjugated structure of rGO sheets will be restored owing to the partial removal of the oxygencontaining functional groups at the high temperature and pressure during the process [37]. And of course, the oligomers are more or less formed through the reaction among the molecules of aminophenol. When the oligomers and other impurities are removed, the space between the rGO sheets will be retained or the space will be adjusted by the molecules, so that the microstructure of the cross-linked rGO hydrogels will be improved [28]. From the photographs shown in Fig. S4, it is clear that the volume of hydrogels can be controlled by using different volumes of the GO suspension (Figs. S4A–E). Different from the clear and colorless solution left by preparing rGOH (Fig. S4B), the orange yellow solution is discovered when the samples of rGOHAPs have formed (Figs. S4C–E), which indicate that the reactions among the aminophenol molecules have been carried out during the solvothermal condition. The diameter for the rGOHAPs column is various although the Rm and the volume of the used mixture are the same. For example, the column diameters for o-rGOHAPs, p-rGOHAPs and m-rGOHAPs prepared from 5.0 mL mixtures are about 1.2, 1.6 and 1.8 cm, respectively; the heights range from 0.9 to 1.0 cm; while the rGOH has far small volume (diameter of 0.8 cm and height of 0.6 cm). On the other hand, the mass is about 10 mg for

(1) −1

Where I is the response current (A), υ the potential scan rate (V s ), m the mass of the one electrode (g) and ΔV the difference of potential during the CV tests (V). Moreover, the Cs, energy density (E) and power density (P) can be calculated according to the GCD curves [33]:

Cs = 2(I t )/ m V

(2)

E = (1/8) Cs V 2

(3)

P = E/ t

(4)

Where I is the discharging current, Δt the discharging time, m the mass of the active material in one electrode and ΔV the window of potential difference during discharging. In addition, the capacitance (C) calculated by impedance response can be separated into the real part Cʹ(ω) and imaginary part Cʹʹ(ω) of the capacitance according to literatures [34,35]:

C ( )=

Z ( )/

C ( ) = Z ( )/

Z( ) Z( ) 2

2

(5) (6)

Where ω is the angular frequency, and Zʹ(ω) and Zʹʹ(ω) are the real and imaginary parts of the complex impedance Z(ω) in Nyquist plot. 181

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rGOHAPs, and about 4.5 mg for rGOH. The various volumes and mass for the hydrogels reveal that the addition of isomers of aminophenol has strong influence on the samples structure. Among the rGOHAPs samples, m-rGOHAPs has the largest volume, indicating relatively loose texture (Fig. S4F). The rGO sheets in rGOH show a compact stacking structure with relatively few pores and crumple (Figs. 2A and S5A). However, wellconnected 3D porous networks are observed in the samples of rGOHAPs (Fig. 2B–D), walls of the pores are thinner than those of rGOH, indicating that aminophenol can effectively alleviate the self-stacking of rGO sheets during the synthesis process. Moreover, more crumpled sheet-like morphologies exhibit in rGOHAPs compared with rGOH, which is favorable to the improvement of the performance. Meanwhile, p-rGOHAP shows most crumpled and interconnected porous nanostructure with good uniformity and regularity than o-rGOHAP and mrGOHAP (Figs. S5B–D). The well connected 3D morphology rGOHAPs can effectively prevent the rGO sheets restacking during the stability testing, and is favorable to diffuse ions and optimize the path for electron transfer [24,27], therefore it can be expected that prepared hydrogels exhibit good capacitive performance. The TEM images show that the rGO sheets are randomly aggregated and exhibit the noticeably thin and curled wrinkle, which seem like that rippled silk waves are interconnected with each other (Fig. 3). The rGO sheets in rGOH show a relatively serious stacking and less crumple (Fig. 3A). The o-rGOHAP sample displays some wrinkled structures and has particle-like products dispersed on the rGO sheets (Fig. 3B). However, it is easy to find that the rGO sheets in p-rGOHAP exhibit less stacking and the most visible wrinkled structure (Fig. 3C); and that the sample of m-rGOHAP also exhibits less crinkles than p-rGOHAP although it also has less stacking (Fig. 3D). The above observations reveal

that the isomers of aminophenol have effect on the hydrogels' structure because of the functional groups’ different positions in the benzene ring. The structural difference between rGOH and rGOHAPs may be originated from the heteroatoms or the defects remained in the plane of rGO sheets [26]. It can be believed that the interconnected thin walls in rGOHAPs can facilitate electron transfer and the adequate contact between the samples of and electrolyte, and that the existence of crumpled structure is favorable to prevent the rGO sheets from stacking and facilitate to form an open-framework structure [26,28]. The strong diffraction peak at 11.9° in XRD pattern of GO (the diffraction of GO (001) plane, Fig. 4A–a) has entirely vanished after the GO dispersion are treated under solvothermal condition and the peak related to C (002) appears at about 24.46° for the sample of rGOH (Fig. 4A–b), which demonstrates that the oxygenated groups on the GO sheets have been partially removed [38,39]. The XRD peaks for orGOHAP, p-rGOHAP and m-rGOHAP are mainly found at about 25.79°, which corresponds to an interlayer distance of 3.45 Å (Fig. 4A–c, d and e). It is notable that the interlayer spacing values in rGOHAPs are smaller than that in rGOH (3.64 Å), which may be attributed to the fact that the presence of aminophenol leads to great reduction degree of GO and less oxygen-containing groups result in smaller interlayer spacing [28]. Two prominent peaks at 1349.6 and 1595.7 cm−1, corresponding to the D and G band, have been found in the Raman spectra of the GO and rGOH (Fig. 4B–a, b). It is generally considered that the D band is related to the disordered graphitic lattice (A1g symmetry), while the G band corresponds to the vibration of sp2-hybridized carbon (E2g symmetry) [30]. The AD/AG ratio increases from 1.58 for GO to 1.61 for rGOH, revealing that more defects have been introduced into rGO sheets when GO sheets are reduced under solvothermal conditions. The samples of

Fig. 3. The TEM images of rGOH(A), o-rGOHAP (B), p-rGOHAP (C), m-rGOHAP (D). 182

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Fig. 4. The XRD patterns (A), Raman spectra (B): GO (a), rGOH (b), o-rGOHAP (c), p-rGOHAP (d), m-rGOHAP (e). The FITR spectra (C) and the N2 adsorption and desorption isotherms (D) and the inset shows the related PSD profiles: rGOH (a), o-rGOHAP (b), p-rGOHAP (c), m-rGOHAP (d).

p-rGOHAP and m-rGOHAP show the D and G bands at 1349.6 and 1589.5 cm−1 respectively (Fig. 4B–d, e). Compared with rGOH, the downshift of the G band means a successful N-doping [7]. The AD/AG increases from 1.61 (rGOH) to 2.60 for p-rGOHAP and to 1.92 for mrGOHAP indicate that more defects generated on the rGO sheets because of the effective reaction agent of p- and m-aminophenol. It is interesting to find that the Raman spectrum of o-rGOHAP (Fig. 4B–c) is different from others, indicating that some oligomers of o-aminophenol have formed on the rGO sheets. The peaks at 1148–1195 cm−1 are assigned to C-H bending vibration of quinoid rings, the peak at 1250 cm −1 corresponds to C-N stretching mode of the polaronic units, the peak between 1340 and 1388 cm−1 (C-N+ vibration of delocalized polaronic structures) reveals the cross-linked structures existed. The peaks located at about 1440, 1500 and 1574 cm −1 can be assigned to C=N stretching vibration of quinoid, C=N stretching vibration of phenazine ring, and the C-C/C=C deformation bands of the benzenoid ring, respectively [7,40,41]. The FTIR spectrum of GO (Fig. S6a) shows alkoxy C-O stretching vibration at 1065 cm−1, epoxy groups C-O-C at 1226 cm−1, C–OH deformation vibration at 1398 cm−1, aromatic C=C stretching vibration at 1622 cm−1, C=O carbonyl stretching vibration at 1728 cm−1, and a very broad O-H stretching vibration at about 3430 cm−1 [42]. After being reduced (Figs. S6b–e), the characteristic peaks of C=O, C-OH, CO and C-O-C groups have decreased, which indicate the removal of some oxygen-containing functional groups in GO [26,28]. The peaks at 1454 cm−1 are attributed to the skeletal vibration mode of benzene rings [26]. Furthermore, it can be seen that the intensities of peaks associated with C=O, C=C, C-OH, C-O and C-O-C become weaker even disappear entirely in the rGOHAPs compared with rGOH (Fig. 4C), which confirms that the oxygen-containing groups have been effectively removed by adding aminophenol. For rGOHAPs samples, the peaks related to the out-of-plane C–H deformations of 1, 2, 4-

trisubstituted benzene rings can be found in the region of 900–700 cm −1 [43]. The additional peak at 1630 cm−1 is attributed to the skeletal stretching vibration mode of quinoid rings in phenazine [26,43], and the peak at 1120-1170 cm−1 is ascribed to the C-N stretching vibration [28]. For o-rGOHAP, the peak at 536 cm−1 is attributed to the out-ofplane ring deformation, which may mean the presence of the polymer formed by o-aminophenol [40]. Careful observation also reveals that all peak's intensities for p-rGOHAP sample are weaker than others, which confirms that the reduction degree of GO is relatively larger in the presence of p-aminophenol (Fig. 4C–b, c and d). According to the Raman spectra and FTIR spectra, we can conclude that the prepared hydrogels have different structures due to the different structures of aminophenol isomers. Different from o-rGOHAP in which oligomer of o-aminophenol has deposited on the rGO sheets, the p and m-aminophenol may be bonded to the rGO sheets via covalent bond. The N2 adsorption/desorption isotherms of the samples can be classified as type IV hysteresis behavior (Fig. 4D) based on the IUPAC classification. Reversible multilayer adsorption/desorption takes place at lower pressures and a small H3-type hysteresis loop caused by a capillary condensation appears at the highly relative pressure range, which indicate the presence of porous and slits structure [27]. From the PSD profiles shown in inset of Fig. 4D, it is found that rGOH and rGOHAPs possesses a wide PSD and main mesopores structure. For rGOH, o-rGOHAP, p-rGOHAP and m-rGOHAP, the BET SSA are calculated to be about 149.5 m2 g−1, 244.2 m2 g−1, 378.2 m2 g−1 and 181.0 m2 g−1, and the total pore volumes are about 0.55, 1.12, 1.27 and 0.73 cm3 g−1 respectively. It is believed that large SSA and pore volume are advantageous to the ion diffusion and electron transfer therefore the capacitive performance can be improved [24]. The GO and rGOH's XPS spectra only show the C and O elements existed (Fig. 5A–a, b), but N element is found in the rGOHAPs samples (Fig. 5A–c, d and e). Compared with that in GO, the O1s peak in 183

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Fig. 5. The whole XPS spectra (A) of GO (a), rGOH (b), o-rGOHAP (c), p-rGOHAP (d), m-rGOHAP (e), the C1s XPS spectra of GO (B) and p-rGOHAP(C), the N1s XPS spectra of o-rGOHAP (D), p-rGOHAP (E), m-rGOHAP (F). Table 1 The list of the atomic composition of samples measured by XPS and the relative contents of the different types of N-containing bonds. Samples

GO rGOH o-rGOHAP p-rGOHAP m-rGOHAP tested p-rGOHAP

Elements content (at.%)

Ratios of elements

Relative contents of different N (at.%)

C

N

O

C/O

N/C

Pyridinic-N

Pyrrolic-N

Quaternary-N

71.49 86.00 83.86 83.21 83.85 79.08

0 0 3.73 5.28 3.40 5.26

28.51 14.00 12.41 11.51 12.75 15.66

2.51 6.14 6.75 7.23 6.58 5.05

0 0 4.44% 6.35% 4.05% 6.65%

19.65 16.93 27.46 15.29

49.31 50.26 42.29 45.59

31.04 32.81 30.25 39.12

hydrogels have dramatically decreased (Fig. 5A), which indicates that the rGO sheets have been formed. The high-resolution C1s XPS of GO (Fig. 5B) can be divided into four peaks at 284.7, 286.3, 287.3 and 289.0 eV which correspond to the groups of C=C/C-C, C-O, C=O and COOR, respectively [24,28]. The high-resolution C1s XPS of rGOH and rGOHAPs (Figs. S7A–C and Fig. 5C) contain the four groups presented in the GO and additional peaks at 285.7 eV and 290.8 eV corresponding to the C=N bond and satellite peak (p-p*) of carbon caused by the π–π* transition of the aromatic cores [44,45]. Notably, the peak of C-N is overlaid by C=O, so the peak around 287.5 eV is assigned to C=O and C-N bonds which has increased obviously after N doping [46,47]. The detailed N1s XPS spectra of rGOHAPs (Fig. 5D–F) can be fitted to pyridinic-N, pyrrolic-N and quaternary-N at 398.8, 400.1 and 401.2 eV, respectively [47,48]. Based on the XPS data, the relative contents of elements, atomic ratios of elements and the contents of different types of N are quantitatively depicted in Table 1. It is obvious that the ratio of C/O has increased after the GO sheets are reduced, and the samples of rGOHAPs exhibit higher C/O ratios than rGOH, especially p-rGOHAP exhibits the highest C/O ratio of 7.23 among the rGOHAPs, which indicates that most oxygen-containing groups are removed on rGO sheets. Furthermore, the N content in p-rGOHAP is the highest (5.28%) and the ratio of N/C reaches up to 6.35%. It is observed that the relative content of pyrrolic-N and quaternary-N is larger in p-rGOHAP compared with orGOHAP and m-rGOHAP. Generally, pyridinic-N and pyrrolic-N can

promote the rapid diffusion and transportation of ions and increase the pseudo-capacitance, while quaternary-N can remarkably enhance the electronic conductivity and improve the rate capability [27,41]. The electrical conductivities of the samples have been also measured by using the four-point probe system. It is found that conductivities of rGOH and o-rGOHAP, p-rGOHAP and m-rGOHAP are about 13.3, 57.7, 67.8 and 41.1S m−1, respectively. This result reveals that aminophenol is an efficient reducing agent to improve the conductivity of the rGO-based hydrogels. It is clear that the conductivity of p-rGOHAP is the highest among the rGOHAPs, which confirms excellent reducing ability of p-aminophenol [22] and coincides with the results obtained from XPS measurements. 3.2. The capacitive performances The capacitive performances of the samples are evaluated by employing two-electrode test system. The rGOH displays quasi rectangular CV shape (Fig. S8A) and symmetrical triangle GCD curves (Fig. S8B). Compared the CV curves of rGOHAP with that of rGOH (Fig. S8C), it is found that the CV for rGOHAP possesses higher current response than that of rGOH at low potential difference while almost same current response at high potential difference, which indicates that some pseudo capacitances exhibit in the material [49,50] besides the electrical double-layer capacitance because of the introduction of the N184

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Fig. 6. The CV curves of the capacitor assembled by the o-rGOHAP (A), p-rGOHAP (B) and m-rGOHAP (C) under different scan rates, the plots of Cs versus the scan rates for the rGOH and rGOHAPs (D): rGOH (a), o-rGOHAP (b), p-rGOHAP (c) and m-rGOHAP (d).

(Fig. 7A–C). When the current density is about 20 A g−1 (Fig. S8D), the iR drops show the sequence as following: m-rGOHAP > prGOHAP > o-rGOHAP > rGOH. The Cs data exhibit a decreasing trend when the current densities increase (Fig. 7D), and the Cs values for rGOH, o-rGOHAP, p-rGOHAP and m-rGOHAP are calculated to be about 155.9, 330.1, 407.5 and 212 F g−1 respectively at the current density of 0.5 A g−1, which is consistent with the results obtained from CV measurements. The largest Cs may be attributed to the largest amount of redox sites, electrical conductivity, unique microstructure, and larger interlayer spacing and reduction degree of rGO sheets in prGOHAP. Furthermore, the effect of electrolyte such as concentration and pH on the performances of p-rGOHAP has also been done (Fig. S9). It can be found that the specific capacitance of the p-rGOHAP increases with the increase of the concentration of H2SO4 solution at low scan rates (the Cs increases with the decrease of the pH values at low scan rates) (Figs. S9A and B). By comparison, p-rGOHAP exhibits relatively good rate capability in 1.0 mol L−1 H2SO4 solution. The values of Cs at 1 mV s−1 for p-rGOHAP are found to be 399.1, 413.9, 427.0, 432.9 and 445.2 F g−1 when the concentrations of H2SO4 are about 0.1, 0.5, 1.0, 2.0 and, 4.0 mol L−1, respectively. Furthermore, the CV curves recorded in different H2SO4 solutions at scan rate of 1 mV s−1 also show the redox peaks (Fig. S9C), which reveals that pseudo capacitance exhibits in p-rGOHAP. According to the above experimental phenomena, we can conclude that the capacitive properties of p-rGOHAP are strongly depended on the concentration of H2SO4 solution. The reasons may be that the H2O molecules are strongly bound to the modified rGO

containing groups and phenolic hydroxyl groups in the material. With the increase of the scan rates, the redox peaks appeared in the CV profiles for rGOHAPs become unobvious and the shapes of the CV are far deviated from quasi rectangle because of the ion diffusion limitation (Fig. 6A–C). The Cs values for rGOH, o-rGOHAP, p-rGOHAP and mrGOHAP can reach to 163.7, 338.8, 427.0 and 232.0 F g−1. On the other hand, the Cs values decrease with the increase of the scan rates (Fig. 6D), it is easy to find that p-rGOHAP sample shows slower downward trend than other rGOHAPs, revealing excellent capacitive performance in p-rGOHAP. Generally, the material can store ionic charges via the physical adsorption at the solid-liquid interface by adsorbing/absorbing the solvated electrolyte ions (electrical double-layer capacitance); at the same time, the sample of p-rGOHAP contains functional group such as N-containing groups (pyrolic-N, pyridinic-N and quaternary-N) and phenolic hydroxyl group because of the reaction occurred out between GO and aminophenol in the preparation process, and the pyrolic-N, pyridinic-N and phenolic hydroxyl groups can carry out the redox reaction to provide the pseudo capacitance [50,51]. Besides, the formed quaternary-N during the reaction can improve the electronic conductivity of the rGO; and the interconnected 3D structure can enhance the diffusion of electrolyte ions and facilitate to the power density. Different from the GCD curves of the cell assembled by rGOH, the cells’ GCD curves fabricated by rGOHAPs exhibit slight deviations from linearity shape due to the contribution of pseudo-capacitance. The iR drops at small current densities are negligible, and they increase with the increase of the current densities during charge-discharge process

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Fig. 7. The GCD curves of the cells fabricated by o-rGOHAP (A), p-rGOHAP (B) and m-rGOHAP (C) at different current densities, the plots of Cs versus the current densities (D). rGOH (a), o-rGOHAP (b), p-rGOHAP (c) and m-rGOHAP (d).

Fig. 8. The Nyquist plot of rGOH and rGOHAPs (the inset shows the high-frequency region of the plots) (A), the variation of real and imaginary part of the capacitance versus the frequency (B), the plots of the energy densities versus power densities according to discharged curves (C), The plots of the Cs values versus the cyclic repeated cycles at the scan rate of 200 mV s−1 for rGOHAPs (D). rGOH (a), o-rGOHAP (b), p-rGOHAP (c) and m-rGOHAP (d).

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hydrogels via hydrogen bonds, in this case, the higher the concentration of H2SO4 solution, the easier the adsorbed water on the rGO surface will be replaced by hydrated H+, then the H+ ions freely transport into small pores of the material and migrate freely in its hydronium form within the intra-layer spaces by means of the Grotthuss mechanism (hopping via the hydrogen-bonding network) [49,52]. Therefore, H2SO4 solution with high concentration is advantageous to the use of more active sites and larger Cs at low scan rate. The ion diffusion and the charge transfer kinetics within the porous network of the hydrogels have also been investigated via EIS technique (Fig. 8). Compared with the capacitors assembled by rGOH, the cells assembled by rGOHAPs show the nearly straight lines at the low-frequency region, indicating good capacitive behavior (Fig. 8A). On the other hand, the depressed semicircles at the high-frequency region reflect the charge transfer at the interface of electrode/electrolyte [24]. According to the Z′-intercept of the Nyquist plots, it is clear that the sequence of the equivalent series resistances is m-rGOHAP > prGOHAP > o-rGOHAP > rGOH, which is consistent with the iR drops obtained from GCD curves. From the relationships between the real part of the capacitance and the frequency shown in Fig. 8B, it can be also found that all the rGOHAPs have larger capacitance than rGOH, and that the sample of p-rGOHAP exhibits the largest capacitance. From the frequency f0 corresponded to the 50% of maximum real part of capacitance (Cʹ) and to the maximal imaginary part of capacitance (Cʹʹ), the characteristic relaxation time (τ0) can be calculated [15]. The f0 values for the SCs fabricated by rGOH, o-rGOHAP, p-rGOHAP and m-rGOHAP are found to be about 0.38, 0.21, 0.24 and 0.13 Hz, corresponding to the τ0 values of about 2.63, 4.76, 4.17 and 7.69 s. This result indicates that p-rGOHAP has the best rate capability among the samples of rGOHAPs although it has less rate capability than rGOH due to its pseudo capacitance. In order to evaluate the application possibility of the hydrogels in SCs, according to the data of discharging curves (Fig. 8C), the energy and power densities based on the active materials for the devices assembled by the hydrogels are calculated. It is obvious that the cells assembled by rGOHAPs exhibit higher energy and power density than that by rGOH. For example, at power density of about 124.3 W kg−1, the cells fabricated by rGOH, o-rGOHAP, p-rGOHAP and m-rGOHAP show the energy densities of about 5.39, 11.46, 14.07 and 7.32 Wh kg−1, respectively. It is easy to find that the Cs and energy density of prGOHAP are better than those of some carbon-based materials (Table 2), indicating a potential application [3,24,33,39,45,46,53–58]. The practical application of SCs is also limited by another crucial parameter of cycling performance. It is clear that the capacitances slightly decrease along with the successive cycle (Fig. 8D), and the cells based on o-rGOHAP, p-rGOHAP, m-rGOHAP and rGOH can retain

85.2%, 92.2%, 59.8% and 94.4% of their initial capacitance after 20,000 cycles. It is interesting to find that the cyclic stability of prGOHAP is strongly affected by the pH value of the electrolyte (Fig. S10). For example, the capacitance of the cell has a loss of 21.5% in 14.7% in 2 mol L−1 H2SO4, 7.8% in 4 mol L−1 H2SO4, −1 1.0 mol L H2SO4, and 9.0% in 0.5 mol L−1 H2SO4 after 20000 cycles. This trend may be originated from that high concentration of H2SO4 cause more material's structures damaged during the repeated charging/discharging process due to the degradation of the active sites. When 1.0 mol L−1 H2SO4 solution is as electrolyte, the cyclic stability of p-rGOHAP is similar to that of rGOH, but the samples of o-rGOHAP and m-rGOHAP have poorer stability than rGOH. The good stability of prGOHAP may come from its large SAA and pores volume. In order to obtain the information about the structure's change, the ex situ XPS spectra for p-rGOHAP sample after charged/discharged has been tested (Fig. S11). It is seen from the whole XPS spectra that the content of C decreases and that of O increases, which can be attributed to the occurrence of redox reaction during the charge and discharge process. The high-resolution C1s and N1s XPS show the similar groups to that in Fig. 5, and the relative contents of the groups (Table 1 and Table S1) show that the contents of pyridinic-N and pyrrolic-N decrease and the content of quaternary-N increases after the material has been repeatedly charged-discharged. However, the relative content of C=C/ C-C group becomes less and the oxygen-containing groups (C=O, C-O) have increased. The results indicate that the pyridinic-N, pyrrolic-N and conjugated carbon region may be acted as active sites, and these active sites are damaged during the charging/discharging process, which will lead to a slow decrease of the capacitance when the cyclic stability tests are in progress [51]. 4. Conclusions The rGO hydrogels have been simply synthesized using GO dispersions and isomers of aminophenol as precursors via solvothermal method. The aminophenol acts as not only a structural modifier to regulate the microstructure of hydrogels, but also as nitrogen sources to dope rGO and improve the specific capacitance. The cell constructed by p-rGOHAP has a maximum energy density of 14.07 Wh kg−1 at power density of 124.3 W kg−1, and an energy density of 4.96 Wh kg−1 at power density of 6.78 kW kg−1. It is proved that the modifying by paminophenol can remarkably improve the capacitive performances of rGO hydrogels because of the formed 3D porous structure and the relatively high N content. The product of p-rGOHAPs may be developed as high-performance electrode material for SCs.

Table 2 Capacitive performances for the reported carbon-based electrode materials. Electrode materials NG-FMs o-rGOHG80 MPC-P170-C700 GH-Hz8 N-rGO NGH-HA12 RGO foam rGOHG-1 AGH200 ACAs-4 CNCs-800 PCM-T800 o-rGOHAPT140 p-rGOHAP1:1

Type graphene graphene carbon graphene graphene graphene graphene graphene graphene activated carbon carbon carbon graphene graphene

Electrolyte 25% KOH 1.0 M H2SO4 6 .0 M KOH 5.0 M KOH 0.5 M H2SO4 25% KOH PVA/H2SO4 25% KOH 6.0 M KOH 6.0 M KOH 6.0 M KOH 6.0 M KOH 1.0 M H2SO4 1.0 M H2SO4

Cs (F g−1)

E (Wh Kg −1

188 (5 mV s ) 253 (1 mV s−1) −1 225 (1 mV s ) 220 (1 A g−1) 234 (5 mV s −1) 205 (1 mV s−1) 193(0.1 A g−1) 177 (1 mV s−1) 207 (3 mV s−1) 250 (0.5 A g−1) 248 (1 A g−1) 317 (1 mV s−1) 339 (1 mV s−1) 427 (1 mV s−1)

187

5.75 8.02 5.42 7.66 8.1 6.89 6.7 4.6 7.2 8.49 8.6 6.80 10.05 12.68

−1

)

P (W Kg 250 125 250 717 500 191 26 250 90 455 446 250 960 960

−1

)

Ref. [3] [24] [33] [39] [45] [46] [53] [54] [55] [56] [57] [58] This work This work

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Acknowledgements The work was supported by the National Natural Science Foundation of China (21574076, U1510121, 21501113, 61804091, 61504076 and 21602127) and Shanxi province (2015021129), the Fund for Shanxi “1331 Project” Key Innovative Research Team and Engineering Research Center (PT201807).

[21] [22]

Appendix A. Supplementary data

[23]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.07.005.

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